Magnetic resonance is used in medical imaging for diagnostic purposes. In magnetic resonance imaging procedures, the region of the subject to be imaged is subjected to a strong magnetic field. Radio frequency signals are applied to the tissues of the subject within the imaging volume. Under these conditions, atomic nuclei are excited by the applied radio frequency signals and emit faint radio frequency signals, referred to herein as magnetic resonance signals. By applying appropriate gradients in the magnetic field during the procedure, the magnetic resonance signals can be obtained selectively from a limited region such as a two-dimensional slice of the subject's tissue. The frequency and phase of the signals from different portions of the slice can be made to vary with position in the slice. Using known techniques, it is possible to deconvolute the signals arising from different portions of the slice and to deduce certain properties of the tissues at each point within the slice from the signals.
Magnetic resonance imaging instruments of the types commonly used for medical diagnostic applications include large, precise magnets which are arranged to impose a high magnetic field, typically about one Tesla or more over a relatively large imaging volume typically 10 cm or more in diameter. Certain magnetic resonance imaging static field magnets severely limit access to the subject. For example, a solenoidal air-core superconducting magnet may have superconductive coils surrounding a tubular subject-receiving space. The subject lies on a bed which is advanced into the said tubular space so that the portion of the patient to be imaged is disposed inside of the tubular space. Iron core magnets typically have ferromagnetic frames defining opposed poles and a subject-receiving space lying between the poles. Permanent magnets or electromagnets are associated with the frame for providing the required magnetic flux. Depending upon the design of the magnet, either the superconductive coils or the frame may obstruct access to the patient during operation of the magnetic resonance instrument. Moreover, because the magnetic resonance imaging instruments typically employed in medicine are expensive, fixed structures, there are substantial costs associated with occupancy of the instrument.
Copending, commonly assigned U.S. patent application Ser. No. 09/083,414, filed May 22, 1998, and the corresponding International Application PCT/US98/10623 and International Publication WO 98/52465, the disclosures of which are hereby incorporated by reference herein, describe apparatus which incorporates magnetic resonance imaging capability in a relatively small device. Apparatus according to preferred embodiments disclosed in these applications includes a movable static field magnet adapted to apply a static magnetic field in a magnetic resonance volume at a predetermined disposition relative to the static field magnet, and may also include an energy applicator such as a high intensity focused ultrasound or "HIFU" device. The preferred apparatus according to the copending applications also includes positioning means for moving the static field magnet and the energy applicator to position the magnet and the applicator so that the magnetic resonance volume at least partially encompasses a region of the subject to be treated and so that the energy applicator is focused within the magnetic resonance volume. For example, the apparatus may include a chassis carrying both the static field magnet and the energy applicator, and the positioning means is arranged to move the chassis relative to the subject. The apparatus may further include ancillary equipment such as gradient coils mounted to the chassis or otherwise secured in position relative to the static field magnet for applying a magnetic field gradient within the magnetic resonance volume, as well as radio frequency equipment for applying radio frequency signals to the subject and receiving the resulting magnetic resonance signals. Apparatus of this type may be used to acquire images of small regions within the patient's body, and may also be used to perform therapeutic procedures such as thermal ablation of tumors or other undesired tissues. The therapeutic procedures can be monitored using the magnetic resonance apparatus.
As disclosed in the copending applications, the static field magnet desirably is a "single-sided" static field magnet. That is, the magnet is arranged so that the magnetic resonance volume where the field is suitable for imaging is disposed outside of the static field magnet and spaced from the static field magnet in a forward direction. Thus, the magnet can be placed alongside the patient, without placing the patient into the magnet. The static field magnet most preferably is substantially smaller than the static field magnets utilized in conventional magnetic resonance imaging instruments. For example, the static field magnet may have dimensions of a meter or less and may be light enough to be moved readily by a positioning device of reasonable cost and proportions. Thus, the entire apparatus can be moved as required to position it adjacent to the region of the subject's body which requires treatment. The most preferred apparatus according to these embodiments is small enough and inexpensive enough to be used in a clinical setting such as a physician's office or medical center. Thus, it is feasible to perform magnetic resonance-monitored energy applying procedures in a normal clinical setting. There is no need to occupy an expensive diagnostic magnetic resonance imaging instrument during such procedures.
The copending applications disclose improved single-sided static-field magnets for magnetic resonance. For example, the static field magnet may include a set of concentric superconducting coils mounted in a cryostat. The dimensions of the coils, as well as the current flows in the coils, are selected to provide a relatively small linear field gradient ##EQU1##
i.e., a relatively small, linear variation of field magnitude B with distance in the axial or Z direction of the coils, within a region spaced forwardly from the coils in the axial direction. In this same region, the radial field curvature ##EQU2##
is also relatively small and hence the field gradient in radial or X directions transverse to the axial direction of the coils is also relatively small. For example, the magnet may provide a field with a linear axial gradient and with very small radial gradients over a region having an axial extent of about 1 cm and having a diameter of about 3 cm, this region being centered on a central point about 20-30 cm forward of the coils. The magnet may be used to acquire images within this region. Because the static field magnet imposes a field gradient in the axial or Z direction, the magnetic resonance frequencies of nuclei vary with location in the axial or Z direction. Different two-dimensional sections or "slices" can be selected by tuning the RF apparatus to the magnetic resonance frequency associated with each axial location.
Another magnet for MRI imaging which uses a set of coils concentric with a central axis to generate a static field having a region suitable for imaging surrounding the central axis is shown in U.S. Pat. No. 4,701,736.
Although the concentric coil designs can provide useful magnetic fields for imaging, further improvements would desirable. Typically, the large, unitary coils used in concentric coil designs are superconducting coils. Large superconducting coils are expensive to fabricate. It would be desirable to provide a magnet design which can generate a field equal to or better than the field obtained with concentric coils, but which does not require large, unitary superconducting coils.